Orbiting electron ionization pump having two anodes



June 10, 1969 J. c. MALIAKAL 3,449,627

ORBITING ELECTRON IONIZATION PUMP HAVING TWO ANQDES Filed'June 'a, 1967 United States Patent US. Cl. 315-109 1 Claim ABSTRACT OF THE DISCLOSURE Orbiting electron type of vacuum pump with a double anode structure comprising a first central anode containing a getter source surrounded by a second transparent anode. With both anodes operating (second at slightly lower potential than the first), the first anode potential basically controls getter sublimation rate and the second anode potential basically controls ionization rate.

The present invention relates to improved orbiting electron types of ionization vacuum pumps. These pumps were introduced to the art by R. Herb, et al., US. Patent 3,244,969. Such pumps offer considerable advantages over the more conventional type of ion pump represented by the sputter-ion pumps described by Dr. L. Hall et al. in U.S. Patent 2,993,638. These advantages include greater simplicity of construction, lighter weight, elimination of magnet requirement, and less outgassing surface. However one advantage of the sputter-ion pump is that its rate of production of getter material is pressure dependent and therefore at pressures in the high vacuum range, production of getter material by sputtering is automatically limited to conserve getter material without the necessity for altering the ionization mechanism. On the other hand, in orbiting electron pumps of the prior art the production of getter material by sublimation from a getter source mounted on its anode can only be reduced by altering the electron injection rates and/or energy at the expense of efficiency of the pumps ionization mechanism.

It is therefore an object of the present invention to provide an improved orbiting electron type of pump with means for effecting a reduction of sublimation rate without reducing the efiiciency of the ionization mechanism.

It is a further object of the present invention to provide an orbiting electron pump construction in which the electric field affecting electron orbiting and hence ionization mechanism is unchanged by erosion of the getter source structure during the course of pump operation.

Other objects, features and advantages of the present invention will in part be obvious and will in part appear hereinafter.

In general, the objects of the invention are achieved by the pump construction described in the above abstract. The invention is now described specifically with reference to the accompanying drawing in which FIG. 1 is a sectional view of an orbitron pump with much of the conventional structure eliminated from the drawing or shown in simplified form to highlight the specific structural features of the present pump which differ from prior art pumps and FIG. 2 is a cross-section viewed as indicated by the line IIII in FIG. 1.

FIG. 1. The improved pump is shown at and the conventional features thereof include a cylindrical, metal pump body 12, which serves as an annular outer cathode, a central anode rod 14 with getter slugs 16 mounted along its length. A filament 18 is provided at one end of the pump body to inject electrons into the annular space between the anode and cathode. Under proper conditions of anode to cathode electric field, the electons orbit about 3,449,627 Patented June 10, 1969 the anode and ultimately many of them strike the anode 14 or the slugs 16 with the result that the slugs are heated to sublime vapor of getter material. Also, the electrons collide with gas molecules in the pump to ionize the gas molecules. The ions are drawn to the cathode wall where they are pumped by chemical combination with and/or burial by condensing getter vapors.

The wall 12 is cooled by means of water coolant pipes or an air blower (not shown). There may be a plurality of filaments 18 and they are constructed and mounted as described in the paper of Arden, Herb, Limon and Maliakal, 1 Journal of Vacuum Science 54-61 (December 1964). Termination tubes 20 and reflectors 22 are provided at both ends of the anode. A radial strut indicated schematically by the dashed line 24 supports the far end of anode 14 at inlet 26 of the pump body. The strut should include an insulator to electrically isolate the anode from the cathode. A connecting flange 28 at inlet 26 connects the pump to a vacuum system 30 to be evacuated to or maintained at high vacuum levels. The anode 14 passes through a conventional insulator feedthrough 32 to make contact with its power supply.

According to the present invention, a second anode 34 is provided in addition to the above described first anode 14. The anode 34 has a highly transparent grid form and may have the form of a helix tied to supporting rods (not shown) or an axial set of rings or even a metal cylinder with holes punched in its surface. The anode 34 is supported by a feedthrough rod 36 passing through a feedthrough insulator 38 for connection to its power supply and is supported at the inlet end by an insulating support indicated schematically at 40. The diameter of the anode 34 is made just slightly larger than that of the getter slugs 16. For instance, in a typical pump, the anode 14 would be a A; inch diameter tungsten rod, the slugs 16 would be inch diameter titanium and the anode 34 would have a to inch diameter and be formed of about .050 inch diameter molybdenum wire helix with a pitch of about 4 inch.

In order to accommodate such a large anode assembly consistent with a high rate of electron orbits, it is necessary to structurally modify the pump by making the radial location of the filament much larger than in prior art pumps. That is, the filament is located at a radial distance from the anode 34 corresponding to between about and of the anode diameter. Typically, the cathode is the pump body and this is made larger than is conventional for a given inlet size.

Referring now to FIG. 2 a typical filament spacing is shown. The anode and getter dimensions are as described above. The anode 34 is supported by three posts 341 (not shown in FIG. 1). The filament 18 is a coiled helix of A inch helix diameter made of .008 inch diameter thoria-coated iridium wire. The filament supporting and shielding post 181 is a nickel wire Ms inch diameter and interposed directly between the filament and anodes at a spacing d from the filament of A; inch. The cylindrical pump body 12 which serves as grounded cathode has a diameter of 8 inches although its inlet 26 (FIG. 1) is adapted to use a nominal six-inch flange 28 (FIG. 1). The filament radial location in such a pump, R, is typically 2 inches and the spacing 1 from anode 34 to filament 18, which controls electron orbiting capability, is therefore about 1 /8 inches. The filament location is similar to that disclosed in my copending application Ser. No. 492,057 filed Oct. 1, 1965, the dimensional being between about and D., although a larger preferred spacing 1 is indicated here.

Typical currents and operating potentials for the pump at the high pressure end of its operating range are 10 kilovolts for first anode 14, 7 kilovolts for the second anode 34, filament emission current of milliamperes,

filament bias of 130 volts positive. At the low pressure end of the pumps operating range, the operator reduces the potential of anode 14 to about 7 kilovolts, equal to the potential of anode 34 or less and thereby reducing the rate of electron acceleration within anode 34 and hence reducing the heating of getter slugs 16 by electron bombardment. The filament bias need not be varied at low pressure. Thus needless titanium sublimation can be limited at low pressure conditions without disturbing optimum ionization conditions.

At high pressures the anode 14 bias is adjusted to draw over half the filament emission current. At low pressures, the anode 14 bias is controlled to draw less than half the filament emission current, the remainder being drawn by anode 34. Ionization conditons are substantially unaffected by this current balance since the electrons from filament 18 make several orbits before striking one or the other of the anodes.

What is claimed is:

1. An improved orbiting electron vacuum pump comprising, in combination:

(a) a first anode consisting of an elongated rod-like member with a source of getter material mounted thereon,

(b) a second anode surrounding the first anode consisting of an annular transparent grid-like member,

(c) an annular cathode surrounding the second anode,

(d) an elongated electron emitting cathode disposed in the annular space between said cathode and second anode, and

(e) means for biassing said first and second anodes to a high positive potential with respect to said cathode so that electrons emitted from said filament orbit about said second anode, and for selectively biassing said first anode more positive than said second anode to accelerate electrons which penetrate the transparent anode, to increase sublimation of said getter material by-electron bombardment, and for biassing said first anode about equal or less positive than said second anode to decrease sublimation of getter material.

References Cited UNITED STATES PATENTS 2,993,638 7/1961 Hall .a 230-69 3,244,969 4/1966 Herb et al. 3l37 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R. 

